This invention relates to a signal correction method for use in receiving an asynchronous wireless signal intended for correcting a sampling time and variations in a tuning frequency, which are required at the time of receiving a wireless signal transmitted from a sending end, as well as to a receiver.
For example, when ID data are verified between a communications unit (hereinafter called a “portable unit”) carried by a driver and another communications unit (hereinafter called a “stationary unit”) installed on a vehicle, an immobilizer employed for preventing the theft of a vehicle permits unlocking of a door lock or activation of an ignition switch.
In relation to such an immobilizer, when changes have arisen in an ambient temperature or when a distance between the portable unit and the stationary unit has become longer, a communication error sometimes arises between the units. In such a case, the units fail to verify ID data, raising a problem of inability to unlock a door lock.
In order to solve such a problem, as described in, e.g., JP-A-10-276110 (hereinafter called a “related-art example 1”), there has been proposed and put into practice a method of monitoring an ambient temperature and changing a tuning frequency by adjusting the capacitance of a varicap diode when the ambient temperature has reached a specified temperature, to thereby correct a tuning frequency drift stemming from temperature changes.
As described in, e.g., JP-A-5-37508 (hereinafter called a “related-art example 2”), there has been proposed another method of correcting a tuning frequency drift, by sampling a received signal through division of received data bits into sixteen equal parts.
However, according to the method described in the related-art example 1, correction of a tuning frequency is limited to correction of a drift stemming from temperature changes. Hence, the method yields no effect for sensitivity failures with respect to changes in pulse width stemming from the influence of the surrounding environment.
The method described in the related-art example 2 yields a problem of inability to effect highly-precise follow-up frequency correction if the allowable limit of pulse width encompasses a wide range.